Gas hydrates are crystalline inclusion compounds of gas molecules in water, which form under certain temperature and pressure conditions (low temperature and high pressure). In the process, the water molecules form cage structures around the gas molecules in question. The lattice framework formed solely from water molecules is thermodynamically unstable, the lattice only being stabilized by the inclusion of gas molecules with the formation of an ice-like compound which, depending on the pressure and the gas composition, may exist even beyond the freezing point of water (up to above 25.degree. C.). An overview of the subject of gas hydrates is provided by Sloan, Clathrate Hydrates of Natural Gases, M Dekker, New York, 1990.
Of particular importance in the petroleum and natural gas industry are those gas hydrates which form from water and the natural gas constituents methane, ethane, propane, isobutane, n-butane, nitrogen, carbon dioxide and hydrogen sulfide. Today's production of natural gas, in particular, is presented with a major problem by the existence of these gas hydrates, especially in those cases where wet gas or multiphase mixtures of water, gas and alkane mixtures are subjected to low temperatures and high pressure. In these conditions the formation of the gas hydrates, owing to their insolubility and crystalline structure, leads to blockages in transport facilities. These include pipelines, valves or production facilities in which wet gas or multiphase mixtures are transported over extended distances at low temperatures, as happens especially in colder regions of the earth or on the sea bed.
Given appropriate pressure and temperature conditions, gas hydrate formation may also present problems during drilling to develop new gas or petroleum reservoirs.
To avoid such problems, gas hydrate formation in gas pipelines or during the transport of multiphase mixtures may be suppressed by the use of relatively large amounts (double-figure percentages relative to the aqueous phase) of lower alcohols such as methanol, glycol or diethylene glycol. The addition of these additives has the effect of shifting the thermodynamic limit of gas hydrate formation to lower temperatures and higher pressures (thermodynamic inhibition). The addition of these thermodynamic inhibitors does, however, cause major safety problems (flash point and toxicity of the alcohols), logistics problems (large storage tanks, recycling of these solvents) and correspondingly high cost, especially in offshore production.
Attempts are therefore being made at present to replace thermodynamic inhibitors by the addition, in the temperature and pressure ranges within which gas hydrates are able to form, of additives (amount used &lt;2%) which either delay gas hydrate formation (threshold hydrate inhibitors, kinetic inhibition) or cause gas hydrate agglomerates to be small and pumpable, so that they can be transported through a pipeline (so-called agglomerate inhibitors or anti-agglomerates).
The gas hydrate inhibitors described in the patent literature include not only the known thermodynamic inhibitors, but also a multiplicity of both monomeric and polymeric classes of substances which represent kinetic or agglomerate inhibitors. Patent Application WO 95/17579 describes quaternary ammonium compounds, especially those which carry butyl or pentyl groups on the quaternary center, as gas hydrate inhibitors. Mixtures of such quaternary ammonium salts with corrosion inhibitors and amidic polymers are mentioned in WO 96/04462. WO 96/08636 describes surface-active substances as gas hydrate inhibitors which carry a polar head group and a hydrophobic radical, the hydrophobic radical containing no more than 12 carbon atoms. Examples mentioned are sodium valerate, butanol, butyl sulfate and butylsulfonate, alkylpyrrolidones and a zwitterion of the formula R.sub.2 N(CH.sub.3).sub.2 --(CH.sub.2).sub.4 SO.sub.3.
Many of these additives, however, have so far not been sufficiently effective or not been available in sufficient quantities or only at high prices, or are not toxicologically safe.
It is therefore an object of the present invention to find novel effective additives which retard the formation of gas hydrates (kinetic inhibitors) or keep the gas hydrate crystals small and pumpable (anti-agglomerates). These are intended to replace the thermodynamic inhibitors (methanol and glycols) still being used at present, which give rise to considerable problems in terms of safety and logistics, and are designed to represent a reduced potential hazard to the environment, compared with the quaternary ammonium salts used.